In a manufacturing process of semiconductor devices, for example, a film formation is performed on a semiconductor wafer by using an apparatus that performs a film forming process by a chemical reaction such as a low-pressure CVD (Chemical Vapor Deposition) apparatus. In the low-pressure CVD apparatus, a chemical reaction is induced in the gas phase under reduced pressure by applying a reaction energy, such as heat and plasma, to a source gas introduced into a reaction container and this reaction product is deposited on the surface of the semiconductor wafer thereby forming a thin film.
In such a low-pressure CVD apparatus, a batch-type vertical CVD apparatus capable of processing a plurality of wafers at a time is widely used in terms of high productivity. In the batch-type vertical CVD apparatus, a plurality of wafers is accommodated in a boat, which is a wafer holding unit, in multiple stages in a vertical direction with predetermined intervals. Then, the film forming process is performed in a state where the boat is introduced in the apparatus.
Moreover, for further improving the productivity, it is considered to accommodate two wafers in the boat in a state of being superimposed and held with the back surfaces thereof being in contact with each other and perform the film formation. However, for realizing such a film formation, there is a problem in accommodation of wafers in the boat and carrying-out of the wafers from the boat. Specifically, it is difficult by the present technology to superimpose two wafers without generating scratches and cracks on the wafers. Similarly, stripping and carrying-out of two superimposed wafers from the boat is also difficult by the present technology.
Therefore, a technology capable of realizing an efficient film forming process is desired in a thin film forming apparatus and a thin film forming method.